The laboratory has been focused during the last year on immune-mediated tumor cell destruction. Natural killer (NK) cells and cytotoxic T cells (CTL) lyse virally-infected and tumor cells by 2 main mechanisms:(1) the exocytosis of lytic granules and (2) the expression of cytotoxic molecules like tumor necrosis factor (TNF) or related molecules like Fas-ligand. We have investigated the importance of these mechanisms in cytotoxic immune responses to various mouse and human tumors We successfully isolated CTL specific for the mouse renal cancer Renca. These CTL exclusively used FasL or perforin/granzymes to lyse Renca cells in vitro. Furthermore,the CTL showed some therapeutic efficacy when transfered to tumor-bearing mice in vivo. Interestingly CTL derived from perforin "knockout" mice could efficiently destroy pulmonary metastases of Renca. This suggests that the perforin pathway was not essential for antitumor effects of the CTL, at least in this metastasis model. We are currently investigating mechanisms underlying how metastases of this tumor in the lungs and various other anatomical sites (such as the peritoneum or liver) are eliminated by CTL. We have also been investigating anticancer therapy using the death ligand TRAIL in combination with the proteasome inhibitor PS-341 (Velcade). We have observed that these 2 agents synergize to promote tumor cell apoptosis in a variety of human and murine cancer cell lines. The molecular mechanism whereby this combination promotes apoptosis has been further investigated. Surprisingly this does not seem to involve inhibition of NFkB by the proteasome inhibitor, but rather decreases in levels of the antiapoptotic protein c-FLIP as well as increases in the TRAIL death receptors. Preliminary in vivo studies suggest this combination may have some promise for future therapeutic utililty.Screening of a large number of human tumor cell lines shows that 20-30% are sensitized to TRAIL by PS-341. The molecular basis for sensitivity and resistance is being investigated further.